Okay, a few things to go through here. First off, as Skitso mentioned, symbiotes are currently not counted in the post-GC screen, I'll clarify this and a few other things as I'm not sure whether this was intentional or not. Captured enemies are also not added to the kill indicator (skulls) and it's possible that one of the aliens were killed by local forces or through friendly fire while not being revealed in shroud.
I will bring it up with the team whether a captured alien should also register as a "kill" for exp and kill count as it feels like it probably should as capturing an alien is harder than killing it.